The objective of this protocol is to ascertain skeletal content of aluminum non-invasively from the increment in serum and urinary aluminum following administration of a chelating agent (deferoxamine) in patients with kidney stones on long-term treatment with potassium citrate. The hypothesis to be tested is that abnormal skeletal retention of aluminum doesn't occur during potassium citrate treatment, because the stimulated aluminum absorption from the formation of absorbable aluminum citrate complex is adequately compensated by renal excretion.